This invention relates to a method and apparatus for measuring and controlling shrinkage of compounds. In particular, this invention relates to a method and apparatus for measuring shrinkage of rocket motor propellant that is caused during curing and controlling shrinkage of the propellant by inducing thermal expansion to prevent creation of voids and cracks therein.
Voids and cracks in high energy propellants, such as those used as rocket propellants, are unacceptable. These anomalies, as well as any separation of the propellant from the walls of the rocket motor, cannot be tolerated since they may create catastrophic consequences.
Tests to determine uniformity of propellant compounds used samples of rocket propellant that were cast into polycarbonate beakers. These beakers were placed in an oven that was kept at the same temperature as the ovens that were used to cure the rocket motors. Every day samples were removed from the oven and tested to see if the propellant had started to gel.
One test used a weighted needle that fell into the propellant for fifteen seconds and the depth that the needle went into the propellant was recorded. The assumption was that the cure shrinkage coincided with the gelling of the propellant slurry. Several specimens were tested daily over a number of days. This data was plotted as time v. needle penetration and an educated guess was made regarding the time when the onset of cure shrinkage was to take place. This time was used to initiate a temperature ramp profile in the curing ovens while the motors were baked. This was at best a hit and miss proposition.
Another testing procedure attempted to determine the volume change by measuring buoyancy of the propellant. The propellant was cast into a condom, the condom suspended from a triple beam balance, and the propellant and condom placed in a water bath. As the propellant shrank during curing, the buoyancy would decrease and the balance would register the weight change. In theory everything should have worked. However, while casting the propellant into the condom, it was noticed that the density of the propellant caused condoms to swell to sizes that were generally unmanageable. In addition, the condoms were found to be semipermeable membranes that allowed water to pass into the propellant which would inactivate the isocyanate curative for the binder system in the propellant. This procedure was not satisfactory.
Another test method incorporated a U-shaped tube filled with Mercury and an electronic Linear Variable Differential Transformer (LVDT) to measure volume change. To a degree, the method worked, but there were drawbacks. The sample had to be loaded into the U-shaped tube without introducing voids into the propellant specimen. This turned out to be a potential source for errors. There was also a long and tedious clean up after the test. Furthermore, special precautions had to be taken when working with hot mercury.
Another testing procedure attempted to float the LVDT directly on the uncured propellant, but the weight of the test apparatus usually caused it to sink into the specimen. As a result, the data derived from these attempts were useless.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a more acceptable method and apparatus for measuring shrinkage of propellant compound that is caused during curing and for controlling the thermal expansion and consequent volume of the compound.